1. Field of the Invention
The present invention relates to a compound semiconductor epitaxial wafer, in particular to a compound semiconductor epitaxial wafer having a gallium phosphide arsenide mixed-crystalline epitaxial layer added with nitrogen.
2. Description of the Related Art
Conventional indirect transition III-V compound light-emitting diodes producing most popularly red light but also yielding orange or yellow lights, in particular those based on gallium phosphide arsenide GaAs.sub.1-x P.sub.x (where, 0.45.ltoreq..times.&lt;1.0) employ an epitaxial wafer in which a GaAs.sub.1-x P.sub.x epitaxial layer is grown on a gallium phosphide GaP or gallium arsenide GaAs single-crystalline substrate, and an uppermost area of the epitaxial layer is doped through diffusion of p-type impurity to form a p-n junction, operating as a light emitting region.
Light emission occurs when an electron and a hole injected upon external voltage application recombine within a light emitting region, that is, a p-n junction area. n-Type carrier concentration is referred as one major factor affecting the luminous efficiency, which has conventionally been set at 1.times.10.sup.15 /cm.sup.3 or above.
Japanese Examined Patent Publication No. 1539 in 1983, for example, describes n-type carrier concentration being limited within a range of 3.5.times.10.sup.15 /cm.sup.3 to 8.8.times.10.sup.15 /cm.sup.3 both inclusive.
In another Japanese Examined Patent Publication No. 101589 in 1994, a specific composition is proposed in which carrier concentration in the n-type epitaxial layer be fallen within 1.times.10.sup.15 to 50.times.10.sup.15 /cm.sup.3, and a ratio of transition peak intensity observed in photocurrent spectra in relation to nitrogen concentration be controlled to fall within a range which is limited with an emission peak energy.
The reason why n-type carrier concentration has been set to 1.times.10.sup.15 /cm.sup.3 or above is that lowering carrier concentration of n-type epitaxial layer is difficult, and that most attempt to lower the concentration beyond that level would often result in an extremely low carrier concentration.
To obtain a maximum emission output, it is necessary to choose, before p-type impurity diffusion, an n-type carrier concentration to an appropriate range for an area or around where a p-n junction (referred as a p-n junction formation area, hereinafter) is formed by a diffusion of a p-type impurity from the uppermost surface of the n-type epitaxial layer.
Since a high concentration of n-type carrier in the vicinity of the p-n junction formation area, for example, causes an unstable luminance level due to low crystal quality and a lowered luminance due to increased absorption of the light, it is preferable to maintain the n-type carrier concentration to a lower level. When the n-type carrier concentration becomes extremely low, on the contrary, opportunities of recombination of electrons and holes become fewer and causes a lowered emission output and a higher forward voltage Vf.
In short, it is essential to set an n-type carrier concentration in an appropriate range, since both of the too high or too low concentrations in the vicinity of the p-n junction formation area can ruin the emission output.
Nitrogen concentration is referred as another factor known to govern the luminous efficiency. Since a light emitting diode using indirect III-V semiconductor results in inefficient emission with a p-n junction alone, general applicaion adds nitrogen acting as an emission center called as an isoelectronic trap in the emission region to improve the efficiency. It is thus said that the concentration of nitrogen added as an isoelectronic trap, as well as n-type carrier concentration, are critical factors affecting luminance, wavelength and other emission characteristics.
Conventional luminous efficiency has, however, been restricted to a lower level since available lower limit of n-type carrier concentration was so limitative that difficulties have been met in optimizing its combination with nitrogen concentration.